Generally speaking, a hydraulic excavator includes: a boom that is raised and lowered relative to a turning unit; an arm swingably coupled to the distal end of the boom; and a bucket swingably coupled to the distal end of the arm. A drive system installed in such a hydraulic excavator includes, for example, a boom cylinder that drives the boom, an arm cylinder that drives the arm, and a bucket cylinder that drives the bucket. These hydraulic actuators are supplied with hydraulic oil from pumps via control valves.
For example, Patent Literature 1 discloses a hydraulic excavator drive system in which an arm first control valve and an arm second control valve are used as control valves for an arm cylinder. The arm cylinder is supplied with hydraulic oil from a first pump via the arm first control valve and also from a second pump via the arm second control valve. The hydraulic drive system disclosed in Patent Literature 1 adopts a configuration for switching, in accordance with a load pressure, a route through which the hydraulic oil returns to a tank at the time of performing an arm crowding operation.
Specifically, in the hydraulic excavator drive system disclosed in Patent Literature 1, a relief line is connected to an arm pushing supply line that connects the arm first control valve and the arm cylinder, and the relief line is provided with an adjustment valve. The adjustment valve blocks the relief line when the load pressure at the time of performing an arm crowding operation is low, and opens the relief line when the load pressure at the time of performing an arm crowding operation is high. Accordingly, by setting the opening area at the meter-out side of each of the arm first control valve and the arm second control valve to a small value, an occurrence of cavitation at the head side of the arm cylinder is prevented. Since the relief line is opened when the load pressure is high, the discharge pressures of the pumps will not become higher than necessary, and thus motive power consumption by the pumps is reduced.